An optimal or efficient stride rate is important because the less energy you use running, the longer or faster you can run.

According to the study, years of research ties natural turnover to individual running mechanics. That is, your preferred stride frequency is a product of the stiffness of your legs or the springy quality of your tendons and ligaments. But muscles, of course, greatly influence running mechanics, so researchers at Penn State set out to determine if muscles’ metabolic demands contribute to the stride frequency your body naturally falls into.

“Our goal was to gain a broader understanding of stride rate,” says study author Riley Sheehan, a Ph.D. candidate at Penn State. “While it’s nice to describe what the body does in terms of one single thing like mechanics, it’s likely multifaceted.”

Sheehan and his co-author had 10 healthy adults (five men, five women, average age 21) run on a treadmill. They recorded the runners' stride frequency and metabolic activity (how much energy specific muscles were using), first during flat running to gain a baseline, then at various stride frequencies while they were running downhill.

Downhill running on a 10% grade (-6 on a treadmill) was used because it’s been shown to be the most cost neutral—the energy gained by momentum is balanced by the energy cost of “braking.” By eliminating the biomechanical cost as much as possible, Sheehan says, they were better able to isolate muscle cost.

They found that when stride rate was slower, there was greater muscle activity during the stance (or foot-on-ground) phase of running. When turnover increased, muscles used more energy during the swing phase, leading Sheehan to conclude that stride rate is at least partially a trade-off between greater muscle costs during these two motions.

This conclusion, Sheehan notes, is likely most interesting to science geeks. But, he adds, the research planted the seed for future studies that might illuminate how to save your quads during downhill running.

During downhill running, your quads do eccentric contractions, which is when a muscle contracts while it's lengthened. This can lead to delayed onset muscle soreness. A greater understanding of muscle activity and stride rate could lead to a reduction in such soreness, Sheehan says. That is, research might describe how small changes in downhill stride rate and length could lessen the toll from lots of eccentric contractions, such as during the first several miles of the Boston Marathon.